Polarographic measuring electrode device

ABSTRACT

An electrochemical measuring electrode device (10) for polarographically measuring the partial pressure of oxygen in an electrolytic medium comprises a cathode (14) which is capable of reducing oxygen and defines an exposed oxygen-reducing cathode surface (20), an anode (18), which defines an exposed anode surface which is arranged relative to the exposed oxygen-reducing cathode surface so as to communicate electrolytically therewith through the electrolytic medium, and a membrane (24), which covers the exposed oxygen-reducing cathode surface and further defines an electrolyte chamber (22) in which the electrolytic medium is confined. In order to effectively promote the decomposition of H 2  O 2  generated in the electrolytic medium as an oxygen reduction intermediate and thereby reduce the response time of the electrode, a stable, non-biological catalytic means is provided catalytically communicating with the H 2O  2. The stable, non-biological catalytic means is preferably constituted by platinum black particles which may be received by the membrane at a central region (42) thereof or alternatively be received by a support structure which may further constitute a covering of at least part of the membrane (24). 
     The platinum black particles may be pressed into a foil material constituting a base material of the membrane or alternatively be applied in a suspension to the membrane whereupon the suspension is solidified.

This is a continuation of U.S. application Ser. No. 875,645, filed June18, 1986, now abandoned.

An electrochemical measuring electrode device for polarographicallymeasuring the partial pressure of oxygen, a membrane for anelectrochemical measuring electrode devie and a method of preparing amembrane for an electrochemical measuring electrode device.

The present invention relates to an electrochemical measuring electrodedevice for polarographically measuring the partial pressure of oxygen.It is well-known to measure the partial pressure of oxygen by means of apolarographic measuring electrode device. A particularly relevant typeof polarographic electrode device is the electrode device of the CLARKtype. The expression "the electrochemical measuring electrode device ofthe CLARK type" covers a well-known concept within the measuringelectrode technology. This electrode device was first described by Clarkat an annual meeting held by the American Society for ArtificialInternal Organs in 1956, and it is distinguishable from theelectrochemical measuring electrode devices for polarographicallymeasuring thepartial pressure of oxygen known up until then in that theelectrode device of the CLARK type comprises an oxygenpermeable membraneseparating an anode and a cathode of the electrode device from themeasuring medium, and in that an electrolyte solution is arranged in anelectrolyte chamber defined by the membrane and communicateselectrolytically and chemically with the anode and the cathode.

In accordance with the principles of the electrode device of the CLARKtype, the partial pressure of oxygen of the electrolyte solutionconstitutes a measure of the partial pressure of oxygen of the medium asoxygen diffuses from the medium into the electrolyte solution. Providedthat the supply of oxygen from the medium is sufficiently high andfurther provided that the rate of diffusion of oxygen through themembrane is sufficiently high, the partial pressure of oxygen of theelectrolyte solution is a true measure of the partial pressure of oxygenof the medium.

Normally, the cathode is made of a metal which is capable of catalysingcathodic reduction of oxygen, and the anode is made of an oxidizablemetal. Suitable cathode metals are noble metals such as platinum andgold and a suitable anode metal is silver. Conventionally, theelectrolyte solution is a pH-stabilized chloride ion-containing aqueoussolution, but the solvent may also be an organic solvent such as glycol.The membrane is of a foil-shaped plastic material with appropriateoxygen diffusion properties and is normally made of e.g. polyethylene,polypropylene, polytetrafluoroethylene or suitable combinations thereof.Between the cathode and the anode, the potential difference ofapproximately -0.6 V is maintained in order to permit the reduction ofoxygen at the cathode. As a result of the reduction of oxygen at thecathode, a current is generated, and the current which is called theelectrode response comsequently constitutes a measure of theconcentration or partial pressure of oxygen of the electrolyte solutionand in the electrode device of the CLARK type further a measure of thepartial pressure of oxygen of the medium separated from the electrolytesolution by the membrane.

As the direct contact between the oxygen-reducing cathode surface andthe sample is avoided due to the membrane, the oxygen electrode deviceof the CLARK type is especially applicable for measuring the partialpressure of oxygen of physiological media or liquids. Thus errors ofmeasurement caused by the presence of other substances such as proteins,which otherwise might give rise to the so-called "poisoning" of thecathode, are avoided.

However, when using an electrode device of the CLARK type, othermeasuring technical problems occur. After using the electrode for sometime, the electrode response, when shifting between samples withdifferent oxygen levels, will become slow (the time constant of theelectrode device becomes high). As a result of the slow response, theelectrode device will exhibit hysteresis at some applications such as inautomatic oxygen analysers in which the electrode response after arelatively short time of the order of 1-2 minutes is used forcalculating the oxygen level of an unknown sample. Due to thehysteresis, the measuring result of a given sample will depend on theoxygen content of the previous samples.

The slow response is therefore particularly disadvantageous when theelectrode device is applied in automatic analysing equipment in which avery high analysing frequency or rate is aimed at.

In a notice in Proceedings of the Physiological Society, December 1969,29P-30P, a polarographic oxygen electrode device of the CLARK type isdescribed in which, inter alia, the presence of catalase was observed toapparently reduce some form of oxygen storage in the electrode. Thisindicated that H₂ O₂ accumulation in the electrolyte solution waschiefly responsible.

It may be hypothesized that an accumulation of H₂ O₂ in the electrolytesolution plays a role in the operation of a Clark-type electrode device,and that said accumulation may be explained by the fact that thecathodic reduction of O₂ to H₂ O occurs in a two-step reaction in whichH₂ O₂ constitutes an intermediate. The second step of the reaction, inwhich the intermediate H₂ O₂ is decomposed to H₂ O, is delayed in anaged oxygen electrode device. The two reaction steps are:

    O.sub.2 +2H.sup.+ +2e.sup.- --H.sub.2 O.sub.2              ( 1)

    H.sub.2 O.sub.2 +2H.sup.+ +2e.sup.- --2H.sub.2 O           (2)

The overall reaction is:

    O.sub.2 +4H.sup.+ +4e.sup.- →2H.sub.2 O             (3)

Since the rate constant of the O₂ decomposition according to step (1) islarge, the rate of step (1) is high. The rate of step (2) is determinedby the rate constant of the H₂ O₂ decomposition, and this rate constantappears to be much lower in an ages electrode device than in a non-agedelectrode device. For the aged electrode device, step (2) will determinethe rate of the overall reaction.

In a non-aged electrode device, in which step (2) is not delayed, theconditions of diffusion of oxygen from the sample to the cathodedetermine the rate of the overall reaction. Due to the large rateconstant of step (2), no accumulation of H₂ O₂ is observed in such anon-aged electrode device.

The slow response of step (2) in an aged electrode device has the effectthat in such an aged electrode device, which is in a steady-statecondition at a constant oxygen level, the part of the electrolytesolution adjacent to the cathode will contain a specific amount of H₂ O₂(the equilibrium concentration of H₂ O₂). The equilibrium concentrationof H₂ O₂ stabilizes on a level so that the rate of formation of H₂ O₂according to step (1) and the sum of the rate of decomposition of H₂ O₂according to step (2) and the rate at which H₂ O₂ diffuses away from theelectrolyte solution part adjacent to the cathode and to the adjoiningelectrolyte solution are equivalent.

As can be seen, the response of the oxygen electrode device will be acurrent consisting of two components: One component which isattributable to the oxygen reduction according to step (1), and onecomponent which is attributable to the H₂ O₂ reduction according to step(2).

Consequently, it is evident that in a case in which the surroundingoxygen level changes or varies, e.g. by bringing a sample of a differentoxygen level into contact with the electrode device, the electrodedevice will not give a stable response (equilibrium current) until a newH₂ O₂ equilibrium concentration in the electrolyte solution has beenreached.

Furthermore, it is clear that the calculation of the concentration ofoxygen on the basis of measuring results provided by means of anelectrode device, when the electrode device is not in a state ofequilibrium, i.e. when the electrode response or electrode current isdifferent from the equilibrium current, will provide results which aredifferent from the results provided by means of an electrode devicewhich is in its equilibrium state.

By modifying the electrode device so that the H₂ O₂ concentration iskept at or approximately at zero, H₂ O₂ being spontaneously decomposedto H₂ O +1/2O₂, the partial reaction will be:

    O.sub.2 +2H.sup.+ +2e.sup.- -H.sub.2 O.sub.2               ( 4)

    H.sub.2 O.sub.2 -H.sub.2 O+1/2O.sub.2                      ( 5)

The overall reaction is as shown above:

    O.sub.2 +4H.sup.+ +4e.sup.- →2H.sub.2 O             (6)

As step (5) proceeds rapidly, H₂ O₂ will not accumulate in theelectrolyte solution and the diffusion conditions of oxygen from thesample to the cathode will determine the rate of the overall reaction.Consequently, such a modified electrode device will generate a stableelectrode response soon after a change of the oxygen partial pressure.

The above-mentioned reduction of the H₂ O₂ level in the electrolytesolution by the addition of catalase is not suitable for improving anelectrode device for polarographically measuring the partial pressure ofoxygen in routine clinical analyses, e.g. in the so-called blood gasanalysers installed in hospitals in connection with operating sectionsor in central laboratories.

To have a blood gas analyser accepted by the users, it is of thegreatest importance that maintenance work is moderate and substantiallybased on ready-to-use products, which on the one hand reduces the timespent on maintenance work and on the other hand reduces the risk ofincorrect analyses caused by errors made by an operator in connectionwith maintenance work.

For routine maintenance of an electrode device of the CLARK type in ablood gas analyser, the membrane and the electrolyte solution are to bereplaced approximately once a month. However, an electrolyte solutioncontaining catalase cannot be prepared as a ready-to-use product, as thedecline in the activity of the catalase during the normal storageperiods would be excessive. Furthermore, to add catalase to the freshelectrolyte solution at the site of use immediately before thereplacement of the electrolyte solution is to be considered a measurewhich cannot be accepted, because hospital personnel themselves are notwilling to carry out preparation work such as preparing reagents,electrolyte solutions or other liquid mixtures.

Therefore, the object of the present invention is to provide anelectrochemical measuring electrode device for polarographicallymeasuring the partial pressure of oxygen, particularly anelectrochemical measuring electrode device of the CLARK type or acomponent for a CLARK type electrode device, i.e. an electrode device ofthe CLARK type exclusive of the electrolyte solution and the membrane tobe placed or arranged on said component at the site of use, whichelectrode device of the CLARK type or which component for an electrodedevice of the CLARK type is capable of generating reproducible and fastresponses which are substantially independent of the age and theprevious life of the electrode device and which electrode device of theCLARK type or which component for an electrode device of the CLARK typeis at the same time as easy to maintain as the known electrode devicesof the CLARK type.

This object is fulfilled by an electrochemical measuring electrodedevice of the abovedefined type, i.e. for polarographically measuringthe partial pressure of oxygen in an electrolytic medium and comprising:

a cathode, said cathode being capable of reducing oxygen and defining anexposed oxygen-reducing cathode surface,

an anode, said anode defining an exposed anode surface, said exposedoxygenreducing cathode surface and said exposed anode surface beingarranged relative to one another so as to communicate electrolyticallywith one another through said electrolytic medium, and

a non-biological catalytic means arranged so as to communicatecatalytically with H₂ O₂ generated at said exposed oxygen-reducingcathode surface and effective for promoting the decomposition of said H₂O₂, said catalytic means being substantially inert to the otherelectrode response-determining components to which it is exposed inoperation of the electrode device and being substantially stable underthe chemical and electrochemical conditions to which it is exposed inthe electrode device.

The above-defined electrochemical measuring electrode device accordingto the invention is preferably an electrode device of the CLARK type ofconstitutes a component for an electrode device of the CLARK type asexplained above. When the electrochemical measuring electrode deviceaccording to the invention is an electrode device of the CLARK type, theelectrode device according to the invention further comprises amembrane, said membrane being permeable to oxygen and covering at leastsaid exposed oxygen-reducing cathode surface and said exposed anodesurface and further defining an electrolyte chamber in front of saidsurfaces, and an electrolyte solution, said electrolyte solution beingconfined in said electrolyte chamber and constituting said electrolyticmedium through which said exposed anode surface and said catalytic meanscommunicates with said exposed oxygen-reducing cathode surface.

In the present specification and claims, the term "stable", as used indescribing the catalytic means, is intended to indicate that thecatyalytic means will substantially retain its efficiency for promotingthe decomposition of H₂ O₂ under the conditions to which it is exposedin the electrode device; e.g. when the catalytic means is incorporatedin the membrane or in a supporting structure, as explained below, thecatalytic means should substantially retain its efficiency for promotingthe decomposition of H₂ O₂ during the normal period between replacementsof the electrolyte solution and the membrane.

The term "non-biological" indicates that the catalytic means does notcomprise a catalytic principle which involves an enzyme or anotherprotein or other biologically generated structure comprising peptide orother organic bonds which would confer instability thereto.

The catalytic means is preferably a solid catalytic means which issubstantially insoluble in the electrolyte even under the conditionsprevailing during operation of the electrode and is preferably providedat a position close to the oxygen-reducing cathode surface of theelectrode device.

As will appear from the figures and the appertaining description, thegood response conditions aimed at are obtained with the electrode deviceaccording to the invention. Futhermore, it is extremely easy to preparepreprocessed electrode devices according to the invention orpreprocessed components for use in the electrode devices according tothe invention and no durability problems have been ascertained inconnection with the preprocessed electrode devices or components.

It should be noted that certain inorganic catalysts otherwise known tobe effective for promoting the decomposition of H₂ O₂, have been foundnot to show the essential stability under the chemical andelectrochemical conditions to which the catalyst is exposed in theelectrode device. Thus, colloidal silver and manganese dioxide have beenfound to be dissolved in the electrolyte of CLARK electrodes and therebyapparently to be converted into components which do not act as catalystsfor the decomposition of H₂ O₂. Activated carbon has been found toabsorb oxygen and thereby to interfere with the current generation inthe electrode.

Although the electrochemical measuring electrode device according to theinvention is preferably of the CLARK type of constitutes a component foran electrode device of the CLARK type, i.e. is adapted to be providedwith a membrane and an electrolyte solution as described above, it isbelieved that the teaching of the present invention, i.e. the provisionof a stable, non-biological catalytic means for promoting thedecomposition of H₂ O₂ generated in the electrolytic medium as anoxygen-reduction intermediate is also applicable in connection withelectrochemical measuring electrode devices for polarographicallymeasuring the partial pressure of oxygen which electrode devices are notof the CLARK type, i.e. which electrode devices do not comprise amembrane and are not adapted to be provided with a membrane, but whereaccumulation of H₂ O₂ generated as an oxygen reduction intermediatewould likewise interfere with the measurement.

The catalytic means may be a means comprising a noble metal, that is, ametal which is placed below, or has a higher oxidation potential than,silver in the electrochemical series, and/or a metal of the platinumgroup. In the present context, the term "comprising" is to beinterpreted in the broad sense.

In the preferred embodiment of the electrode device according to theinvention, the catalytic means is constituted by a platinum black means.The platinum black means may be constituted by platinum black particles,and the platinum black particles may be received by the membrane, oralternatively, the electrode device further, preferably, comprises asupport structure, the support structure being permeable to H₂ O₂ andwater, and the platinum black particles being received by the supportstructure. The support structure means may constitute a covering of atleast part of the membrane. It is preferred that the cross-sectionalarea of the individual platinum black particles is substantially smallerthan the exposed oxygen-reducing cathode surface area. In particular, itis preferred that the particulate platinum black is substantiallyconstituted by particles, the maximum diameter of which is less than 1μm.

The platinum black particles received by the support structure orreceived by the membrane may preferably be located at the side of themembrane facing the electrolyte chamber. Although the catalytic meansconstituted by the platinum black particles is preferably received by asupport structure preferably constituting a covering of at least part ofthe membrane, the catalytic means constituted by the platinum blackparticles may, alternatively, be constituted by a separate catalyticmeans componet which is received in the electrode device according tothe invention as defined above, however e.g. protrudng into theelectrolyte chamber or located adjacent to or in front of theoxygen-reducing cathode surface. By providing the catalytic means of theelectrode device as platinum black particles received by the membrane orreceived by the support structure preferably constituting a covering ofat least part of the membrane, it is ensured that the effectivepromotion of decomposition of H₂ O₂ takes place at the appropriate placein relation to, i.e. adjacent to or in front of the exposedoxygenreducing cathode surface.

To ensure effective H₂ O₂ reduction in the entire section of theelectrolyte chamber communicating with the cathode surface, the platinumblack particles are preferably in accordance with the present inventionprovided in such an amount and arranged relative to the exposedoxygen-reducing cathode surface in such a manner that the concentrationof H₂ O₂ is maintained substantially at zero in any volume of theelectrolyte in the electrolyte chamber electrolytically communicatingwith the oxygen-reducing cathode surface, e.g. in a part of theelectrolyte chamber having the oxygen-reducing cathode surface as acenter and being within a distance from the cathode surface of max. 100times the cathode surface diameter.

In connection with an oxygen electrode device according to the inventioncomprising a 20 μm diameter platinum cathode and a 20 μm polypropylenemembrane, arranged at a distance of approx. 0-5 μm from the exposedoxygen-reducing cathode surface, it is preferred that the platinum blackparticles received by said support structure constituting a covering ofat least part of the membrane are present in a surface density of 1-6000μg/cm², preferably 20-600 μg/cm².

The platinum black particles may be arranged in direct contact with theelectrolyte solution. Alternatively, the platinum black particles may beincorporated in a material which is permeable to H₂ O₂ and water toavoid the electron-conducting contact between the platinum blackparticles and the cathode. Suitable materials are polyurethane,cellulose acetate, polyvinyl acetate and cellophane.

The electrode device according to the present invention may, e.g., be anelectrode device of CLARK type constituting a component of, e.g., ablood gas analyser for analysing samples of whole blood, or it may,e.g., be a transcutaneous electrode device. The transcutaneous electrodedevice will preferably further comprise means for thermostaticallyheating the electrode device to a predetermined temperature, said meanscomprising a temperature sensor means and a heating means.

The present invention also relates to a membrane for an electrochemicalmeasuring electrode device for polarographically measuring the partialpressure of oxygen in a medium, said membrane being permeable to oxygenand being provided with a coating of at least part of the surface ofsaid membrane, said coating comprising a non-biological catalytic meanseffective for promoting the descomposition of H₂ O₂, said cayalyticmeans being substantially inert to the other electroderesponse-determining components to which it is exposed in operation ofthe electrode device when the membrane has been mounted on the electrodedevice and being substantially stable under the chemical andelectrochemical conditions to which it is exposed in operation of theelectrode device.

Preferred embodiments of the membrane according to the invention arediscussed above in connection with the general description of theinvention.

The present invention further relates to methods of preparing a membranefor an electrochemical measuring electrode device for polarographicallymeasuring the partial pressure of oxygen, the membrane being of aplastic material permeable to oxygen. In accordance with a first methodaccording to the present invention, the method comprises the followingsteps:

providing a foil of said plastic material,

providing a suspension of platinum black particles in an organic liquid,

applying said suspension to a side surface of said foil of said plasticmaterial,

evaporating said organic liquid,

heating said plastic material to a temperature in excess of itssoftening temperature, and

forcing said platinum black particles into said plastic material foilbeing softened by applying mechanical pressure to said platinum blackparticles.

In accordance with another or alternative method according to thepresent invention, the method comprises the following steps:

providing a foil of said plastic material,

providing a suspension of platinum black particles in a plastic materialin liquid state,

said material being permeable to oxygen, H₂ O₂ and water,

applying said suspension to a side surface of said foil of said plasticmaterial so as to provide a homogeneous liquid covering of said sidesurface of said foil, and

solidifying said plastic material so as to establish a solidifiedcovering of said side surface of said foil.

In the above-described first method according to the present invention,the organic liquid may e.g. be benzene or ethanol. In the alternativemethod, the plastic materials may, e.g., be dissolved in a solvent,and/or it may be in a monomeric or oligomeric state which is polymerizedto form the solidified covering.

The platinum black particles constituting a stable, non-biologicalcatalytic means effective for promoting the decomposition of H₂ O₂generated by the electrochemical measuring electrode device as an oxygenreduction intermediate may alternatively be applied to the membraneaccording to the invention by other techniquies, such as printingtechniques, rolling techniques or, quite generally, techniques involvinggluing the platinum black particles to the plastic material of themembrane.

The invention will now be further described with reference to thedrawings, in which

FIG. 1 is a schematical view of a first, presently preferred embodimentof an electrochemical measuring electrode device of the CLARK type andaccording to the invention for measuring the partial pressure of oxygen,

FIG. 2 is a perspective and partly broken away view of a front part ofthe electrochemical measuring electrode device shown in FIG. 1 alsoillustrating a first embodiment of a membrane according to theinvention,

FIG. 3 is a perspective and partly broken away view corresponding to theview of FIG. 2 also illustrating a second and presently preferredembodiment of the membrane according to the invention,

FIG. 4 is a perspective and partly broken away view of a secondembodiment of the electrochemical measuring electrode device accordingto the invention, the electrode device being a transcutaneous,polarographic and potentiometric electrode device,

FIG. 5 is an elevational view of the first embodiment of the membraneaccroding to the invention shown in FIG. 2,

FIG. 6 is an elevational and partly broken away view of the second andpresently preferred embodiment of the membrane according to theinvention shown in FIG. 3,

FIGS. 7, 8 and 9 are schematical views illustrating separate steps of afirst method according to the invention of preparing the firstembodiment of the membrane according to the invention,

FIGS. 10 and 11 are schematical views of two alternative embodiments ofthe method according to the invention of preparing the second andpresently preferred embodiment of the membrane according to theinvention,

FIG. 12 is a diagrammatical view illustrating the responses of anelectrode device of the CLARK type according to the invention and theresponse of a conventional electrode device of the CLARK type, theelectrode devices being identical to one another except for thecatalytic means characteristic of the present invention, and

FIG. 13 is a diagrammatical view illustrating the responses of theelectrode devices mentioned above with reference to FIG. 12 in adifferent test set-up.

In FIG. 1, a first embodiment of an electrochemical measuring electrodedevice of the CLARK type according to the invention for measuring thepartial pressure of oxygen in a blood sample is shown. The electrodedevice is designated by the reference numeral 10 in its entirety andcomprises a plastic housing 12, a platinum cathode 14 which isconstituted by a platinum wire of a diameter of 20 μm, which is sealedinto a lead glass tube 16, and an anode 18. The platinum cathode 14defines an active, exposed oxygen-reducing cathode surface 20, which isin contact with an electrolyte solution, which is confined in anelectrolyte chamber 22 defined by an oxygen-permeable plastic membrane24. As is evident from FIG. 1, the plastic housing 12 encircles the leadglass tube 16 so that an annular chamber 26 is defined between theplastic housing 12 and the lead glass tube 16. The annular chamber 26communicates with the electrolyte chamber 22 through an electrolytesolution passage 28. The anode 18 is constituted by a silver/silverchloride coating of the lead glass tube 16 and communciates chemicallywith the electrolyte solution of the annular chamber 26 and further withthe electrolyte solution of the electrolyte chamber 22 through theelectrolyte solution passage 28.

On the membrane 24, platinum black particles of a particle size of lessthan 1 μm are applied, as shown in FIG. 2 or, alternatively as shown inFIG. 3. The membrane 24 is maintained in position in front of the endsurface of the electrode device 10 by means of an O-ring 30 and coversthe exposed oxygen-reducing surface 20 of the Pt-wire cathode 14. Theelectrolyte solution is an aqueous solution of the composition: 191mmole KH₂ PO₄, 298 mmole Na₂ HPO₄ 2H₂ O, 139 mmole KCl, 0.26 mmole AgCland thymol as a germicide. In use, a polarization voltage of -630 mV issupplied to the electrode device 10 from an external measuring apparatus32 through a cable 34.

The electrode device is fitted in a measuring cell 36 of the typedescribed in U.S. Pat. No. 4,160,714. In the measuring apparatus 32, thecurrent generated by the electrode device by the reduction of oxygen isamplified and converted from analog to digital form in an A/D converterfor calculating the partial pressure of oxygen, which is displayed on afirst display 38 of the measuring apparatus 32. The measuring apparatus32, further comprises a second display 40 for displaying the currentgenerated by the electrode device 10 in digital representation.

In FIG. 2, the end part of the electrode device 10 according to thepresent invention is shown in greater detail also disclosing thestructure of the membrane 24 which comprises a non-biological catalyticmeans constituted by platinum black particles effective for promotingthe decomposition of H₂ O₂ generated in the electrolyte solution by thereduction of oxygen. Although the catalytic means is preferablyconstituted by a covering of at least part of the membrane of theelectrode device, the catalytic means may, alternatively, e.g., be alayer in an annular configuration or an annular groove arranged at thefront end of the lead glass tube 16, e.g., encircling the cathode.

The membrane 24 is a circular piece of biaxially orientatedpolyproplyene foil of a thickness of 20 μm. The diameter of the membrane24 is 13 mm and the central part of the membrane 24 contacting a sampleis approximately 5 mm. In a central region 42 of the membrane 24,platinum black particles are pressed into the foil of the membrane. Thecentral region 42 measures approximately 7 mm².

In FIG. 3, which is a partly broken away view of the end portion of theelectrode device 10 basically corresponding to the view of FIG. 2, analternative and presently preferred embodiment of the membrane 24according to the invention is shown. The membrane 24 shown in FIG. 3 isa two-layer membrane, a first or outer layer of which is designated bythe reference numeral 44 and constituted by the above-describedbiaxially oriented polyropylene foil. An inner layer 46 of the membrane24 is constituted by a solidified plastic material including platinumblack particles constituting a stable, non-biological catalytic meanseffective for promoting the decomposition of H₂ O₂ generated in thereduction of oxygen.

The preparation of the first embodiment of the membrane 24 shown in FIG.2 and the preparation of the second embodiment of the membrane 24 shownin FIG. 3 will be described in greater detail below with reference toFIGS. 7-9 and 10-11, respectively. In FIGS. 5 and 6, the first and thesecond embodiment, respectively, of the membrane 24 are shown inelevational views and in FIG. 6 in partly broken away view.

In FIG. 4, a second or alternative embodiment of the electrode device 10according to the invention is shown. The electrode device shown in FIG.4 is a transcutaneous and combined polarographic and potentiometricelectrode device of the type described in the applicant's publishedinternational patent application PCT/DK81/00035, publication No. WO81/02831 to which reference is made. In FIG. 4, components similar tothe components described above with reference to FIGS. 1-3 aredesignated by reference numerals identical to the reference numerals ofFIG. 1-3. In the transcutaneous, polarographic and potentiometricelectrode device 10 shown in FIG. 4, the anode 18 is constituted by asolid silver body. Centrally within the solid silver body, apH-electrode 48 is arranged. At the lower side surface of the silverbody 18, recesses 50 are provided which are filled by the electrolytesolution and constitute electrolyte reservoirs. In two further recessesof the silver body 18, a Zener diode 52 and an NTC resistor 54 arearranged. The Zener diode 52 and the NTC resistor constitute a heatingelement and a temperature detector element, respectively, of thetranscutaneous polarographic and potentiometric electrode device, as iswell-known in the art. In front of the pH-electrode 48 and further infront of the cathode of the electrode device, not shown in FIG. 4, aspacer structure 56 is arranged. The spacer structure is constituted bya water and H₂ O₂ and carbon dioxide permeable plastic element furthercomprising platinum black particles constituting a catalytic meanseffective for promoting the decomposition of H₂ O₂ formed by thereduction of O₂. As is evident from FIG. 4, the spacer structure 56 isof a basically annular configuration.

The platinum black particles of the membrane 24 or of the spacerstructure 56 are prepared by grinding commercially available platinumblack of the type FLUKA No. 81110. The commercially available platinumblack has a variable grain size of approx. 5 μm.

When preparing the ground platinum black particles, 1 g is weighed outand suspended in benzene at a concentration of approx. 20 mg/ml. Thesuspension is transferred to a beaker filled with small glass beads, andstirred on a magnetic stirrer of a conventional laboratory type for 24hours. Thus a particularly finely distributed platinum black (dust) isobtained with a comparatively uniform particle size distibution and withan average particle diameter <0.5 μm.

The platinum black dust is easily suspended in the suspending agent.

In accordance with a first method of preparing a membrane according tothe invention, 10 μl of a benzene or ethanol suspension of theabove-described platinum particles is, as shown in FIG. 7, taken out bymeans of a Carlsberg pipette 58 and transferred to a circularpolypropylene foil 60. As is evident from FIG. 7, the foil 60 may besupported on a support structure 64 which further includes a heatingelement 66. In FIG. 7, the 10 μl suspension including the platinum blackparticles is designated the reference numeral 62. The suspending agentis evaporated by standing for about 5 minutes. Alternatively, theevaporation could be performed by applying heat to the support structure64 from the heating element 66. Thereafter, as is evident from FIG. 8, aTEFLON® foil 68, the melting point of which exceeds 175° C., is arrangedon top of the foil 60 with the layer of platinum black particles 62 fromwhich the dispersing agent has been evaporated. A pressing tool 70 whichalso includes a heating element 72 is brought into contact with theupper side surface of the TEFLON® foil 68 as indicated by an arrow 73and pressed against the foil 60 with a pressure of approximately 300kg/cm² (29.4×10⁶ N/m²), while heat is applied to the pressing assemblyfrom the heating elements 66 and 72 in order to heat the foils 60 and 68to a temperature of approximately 130° C. The above pressure ismaintained for a period of time of approximately 10 min., whereupon theplatinum black particles have been pressed into the foil material 60.

After the above 10 min. period of time, the heating elements 66 and 72are disconnected from their supply. When the system has cooled to roomtemperature, the pressing tool is elevated and, as is evident from FIG.9, the TEFLON® foil 68 is removed. The platinum black particles adherecompletely to the polypropylene foil 60.

From an electrode response time point of view, the above-described firstmethod of preparing a membrane according to the invention may be themost advantageous as the membrane prepared in accordance with theabove-described method basically has characteristics as to strength andpermeability to oxygen which are identical to the characteristics of apolypropylene membrane which does not include platinum black particles.However, it has been found that this method may sometimes result in amembrane which may give rise to a quiescent or zero current of theelectrode device when the membrane is arranged on the electrode deviceand maintained in poisiton by means of its O-ring as shown in FIGS. 1-3.This is believed to be due to the fact that some of the platinum blackparticles which should have been pressed into the membrane materialprotrude from the inner side surface of the membrane and areconsequently brought into contact with the exposed cathode surface sothat a galvanic current is generated by the contact between the exposedcathode surface and the platinum black particles. Thus, in this method,it seems important to ensure that substantially all of the platinumblack particles are pressed into the membrane material.

In accordance with a second and presently preferred embodiment ofpreparing the membrane according to the invention, the platinum blackparticles are suspended in a plastic material which is in a liquidstate, e.g., in solution, whereupon the suspension is applied to thepolypropylene foil and solidified, e.g., by evaporation. Twoalternatives of applying the suspension to the foil in accordance withthe above-described presently preferred embodiment of the methodaccording to the invention are shown in FIGS. 10 and 11.

In FIG. 10, the suspension of the platinum black particles in the liquidplastic material is poured onto an outer or upper side surface of thepolypropylene foil 60 which is arranged in an oblique position,whereupon the suspension which is designated by the reference numeral 74in FIG. 10 flows down the oblique foil whereby a homogenous suspensionlayer is provided and is solidified, e.g., by evaporation.

The suspension 74 may, as is shown in FIG. 11 alternatively be appliedto the outer or upper side surface of the foil 60 in a screening processby employing a squeegee or spatula 76.

The liquid plastic material may, e.g., be prepared as a solution of 0.35g PUR (polyurethane) in 5 ml DMF (dimethylformamide), and 20 ml THF(tetrahydrofuran) which may be prepared within a few hours. In 3 ml ofthis solution, 5 mg platinum black particles are suspended and ground bystirring with a magnetic stirrer for about two hours in a beaker ofdiameter 20 mm filled with about 20 glass beads of a diameter of 3 mm.

As will be understood, the response time of the electrode devicecomprising the above described second and presently preferred embodimentof the membrane accordint to the invention may be slightly slower thanthe response time of the electrode device comprising the above-describedfirst embodiment of the membrane according to the invention if themembrane thickness is not reduced to compensate for the added layer.However, the increase in the response time by applying the inner layer46 to the outer layer 44 of the membrane 24 shown in FIG. 3 only amountsto approximately 50%, and the response time of the electrode deviceshown in FIG. 3 may obviously be reduced by simply decreasing thethickness of the outer membrane layer 44. The membrane 24 shown in FIGS.3 and 6 has the advantage that the platinum black particles are allembedded in the layer 46 and thus cannot give rise to a quiescentcurrent or zero current.

In FIG. 12, a diagram is shown illustrating the response A of an agedoxygen electrode device as shown in FIGS. 1 and 2 according to theinvention comprising a membrane of the type described above withreference to FIGS. 2 and 5 and prepared as described above withreference to FIGS. 7-9 and the response B of the same electrode devicecomprising a polypropylene membrane of a conventional type, i.e. notincluding platinum black particles.

The responses were recorded in an experiment in which the electrodedevices were mounted in a set-up of the type shown in FIG. 1 formeasuring the oxygen partial pressure on gas samples. Atmospheric airsaturated with aqueous vapour, i.e. air of an oxygen partial pressure of145 mm Hg (19.3 kkPa) was introduced into the measuring cell 36 every 6minutes and conducted past the electrode device.

After the oxygen electrode device has been mounted in the measuring cellfor approx. 3 days, a sample of the composition of 95% argon, 5% CO₂,i.e. of an oxygen partial pressure of zero, was introduced into themeasuring cell. The digitized response of the electrode device wasrecorded every 2 seconds and converted to mm Hg on the basis ofcalibration data at 145 mm Hg. The two response curves A and B shown inFIG. 3 were plotted on the basis of the response data thus recorded.

It is seen that after 90 seconds the electrode current and/or theresponse B of the conventional electrode device reaches a valuecorresponding to an oxygen partial pressure of approximately 9 mm Hg,whereas the electrode current of the response A of the electrode deviceaccording to the invention already after 45 seconds reaches a stablevalue corresponding to an oxygen partial pressure of 2 mm Hg. Thus itcan be seen that the response of the electrode device of the presentinvention reaches its stable level considerably faster than theconventional electrode device.

Based on the theoretical reflections above, it is evident that theresponse curve B of the conventional ages electrode device will bepositioned above the response curve A of the electrode device accordingto the invention, when the oxygen level in the sample is lower than theoxygen level at which the electrode device has stabilized. This is dueto the fact that the H₂ O₂ level in the electrolyte solution while theresponse curve is recorded is higher than the level of H₂ O₂ originatingfrom the present oxygen level and thus provides a higher current thanthe current to be expected from the present oxygen level.

The rate of the response is usually expressed by the time constant ofthe electrode device, which is approximately 5 seconds for a non-agedconventional oxygen electrode device. The same value is ascertained forthe electrode devices according to the invention. Aged conventionaloxygen electrode devices have a small time constant during the firstpart of the response and a large time constant during the last part ofthe response.

In connection with measurements on samples with an oxygen level lowerthan the oxygen lever at which the electrode device has stabilized (145mm Hg), the time constant during the last part of the response for agedconventional oxygen electrode devices was experimentally found to bemore than 15 seconds.

On the basis of the response curves, it is estimated that the analysingtime may be reduced by more than 50% in an analysing apparatus in whichthe electrode device according to the invention replaces theconventional device.

In FIG. 13, the response D of an aged electrode device according to theinvention and of the type described above with reference to FIG. 12 andthe response C of the conventional eletrode device also described abovewith reference to FIG. 12 are shown.

With the one exception that a sample of a high oxygen level (80% O₂, 5%CO₂, 15% N₂) was used instead of a sample of an oxygen level of zero,the experimental work which led to the plotting of FIG. 13 was carriedout in the same manner as described in connection with FIG. 12.

In this case a lower oxygen content was recorded (as expected) whenemploying the conventional electrode device than when employing theelectrode device according to the invention.

We claim:
 1. A polarographic electrode device of the Clark type forpolarographically measuring the partial pressure of oxygen,comprising:(a) a cathode capable of reducing oxygen and defining anexposed oxygen-reducing cathode surface; (b) an anode defining anexposed anode surface; (c) an O₂ -permeable membrane; (d) an electrolytechamber defined at least in part by said O₂ -permeable membrane, andcontaining therein said exposed oxygen-reducing cathode surface and saidexposed anode surface; (e) an electrolyte solution confined in saidelectrolyte chamber and providing an electrolyte medium through whichsaid anode surface and cathode surface communicate with one another; and(f) non-silver noble metal particles contained in said electrolytechamber arranged so as to be in communication with the electrolytesolution and adjacent to the exposed oxygen-reducing cathode surface. 2.An electrode device according to claim 1, in which the particlescomprise a platinum group metal.
 3. An electrode device according toclaim 2, in which the particles comprises platinum black particles. 4.An electrode device according to claim 3, in which said platinum blackparticles are received by said membrane.
 5. An electrode deviceaccording to claim 4, in which the platinum black particles are presentin a surface density of 1-6000 μg/cm².
 6. An electrode device accordingto claim 3, in which the cross-sectional area of the individual platinumblack particles is smaller than the exposed oxygen-reducing cathodesurface area.
 7. An electrode device according to claim 6, in which theplatinum black particles are substantially constituted by particleshaving a maximum diameter of less than 1 μm.
 8. An electrode deviceaccording to claim 3, which further comprises a support structure, saidsupport structure being permeable to H₂ O₂ and water, and said platinumblack particles being received by said support structure.
 9. Anelectrode device according to claim 1, further comprising means forthermostatically heating the electrode device to a predeterminedtemperature and comprising a temperature sensor means and a heatingmeans.